Electronic structure and fragmentation properties of [Fe4S4(SEt)4−x(SSEt)x]2−

A limited exposure of (n-Bu4N)2[Fe4S4(SEt)4] solutions in acetonitrile to air was found to produce a new series of [4Fe–4S] cluster complexes, [Fe4S4(SEt)4−x(SSEt)x]2− (x = 1–4), with the original –SEt ligands substituted by –SSEt di-sulfide ligands, which were formed due to partial decomposition of the [4Fe–4S] core in parent [Fe4S4(SEt)4]2−. The products were first observed in the experiments with an ESI-ion Trap-TOF mass spectrometer and were further identified using high resolution Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. Photoelectron spectra of the [Fe4S4(SEt)4−x(SSEt)x]2− dianions revealed that the –SSEt coordination induced little change in the electronic structure of the [4Fe–4S] cluster, but the electron binding energies of [Fe4S4(SEt)4−x(SSEt)x]2− increased from 0.52 to 0.73 eV with increase in x from 0 to 4, suggesting a greater electron withdrawing ability of –SSEt than –SEt. In high resolution MS/MS experiments on [Fe4S4(SEt)3(SSEt)]2−/1−, clusters with both charge states yielded fragment [Fe4S4(SEt)3]−, suggesting that –SSEt could be lost either as a negatively charged ion SSEt− from the doubly charged precursor, or as a radical SSEt from the singly charged species. The biological implication of the interaction between [Fe4S4(SEt)4]2− and O2 is discussed in comparison to the air exposure of [4Fe–4S] proteins to the air.